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Each of the receivers for the new Berkeley Illinois Maryland Array (BIMA) millimeter wave telescopes is designed to accomodate 4 SIS mixers, covering the 70-90, 90-115, 130-170, and 210-270 GHz frequency bands. For simplicity and compactness, and because they must be remotely controlled, the receivers are single polarization, with the 4 fixed-tuned waveguide mixers housed in one dewar and cooled by a single 4 K closed cycle refrigerator. The Gunn diode local oscillators are coupled quasi-optically. The receiver design is modular; for example, entire local oscillator assemblies are easily interchanged. A complete receiver weighs about 60 kg including local oscillators and the refrigerator coldhead. The dewar is about 25 cm diameter and 55 cm long.
The receiver is mounted at the Cassegrain focus with each of the scalar feed horns located slightly off the telescope axis. The straight-line optical path has no movable elements; instead, changing frequency bands is accomplished by offsetting the antenna pointing slightly. The coma due to the off-axis feed location gives an rms wavefront error of less than 1/550 wavelength even at 270 GHz. Aspheric teflon lenses along the optical path form an image of the telescope at the aperture of each feed horn, providing frequency independent coupling. The lenses also serve as the dewar vacuum window and radiation shield windows, giving high transmission at mm wavelengths along with adequate blocking of infrared radiation. The measured telescope background temperature is only 6 K due to the clean straight-through design of the telescope and receiver optics. Aperture efficiencies are expected to be 70\% to 75\% at all frequencies.
This work was supported by NSF grant AST91-00307.
